1. Field of the Invention
The present invention relates in general to tape storage devices and in particular to a method and apparatus for detecting and managing skew in a multitrack environment.
2. Description of the Prior Art
Multitrack tape recording involves the writing and reading of data on more than one track simultaneously. In the data processing industry, tape devices used as secondary storage, operate almost exclusively in the multitrack environment. While multitrack recording and reading provides a high data transfer rate, it is not without its disadvantages. These disadvantages are heightened by high tape speeds, high tape capacities and decreasing thickness of tapes used in current tape drives.
When high density data recording in a multitrack environment is performed, severe edge track to edge track skew can result. This skew can hinder or even prevent the recovery of data stored on the tape. Skew is caused by several factors inherent in the tape drive environment. First, in multitrack tape drives it is necessary to align the multiple head elements with each of the data tracks on the medium. This physical alignment must typically be repeated periodically to ensure consistent and accurate operation. The problem with head adjustment, however, is that it can introduce skew. By adjusting the head to line up with each of the tracks on the tape medium, a resultant skew between tracks can occur.
Skew is similarly introduced by such factors as a large separation between the concurrent tracks, the use of a flexible tape medium, and variations in physical tolerances between the head elements and the particular seating of the tape cartridge within the drive. Finally, the high linear density in modern tape formats magnifies all of these factors to further increase the magnitude of the skew. Interchange (where a tape is written on one drive and read on another) can, and often does, aggravate the problems associated with tape skew.
Skew is manifested, on the average, as a uniformly sloped misalignment of data as they are read from tape. The effects of skew prevent normal track voting and hinder the ability to reacquire synchronization of lost tracks. When reading the block, at initial acquisition, the uncertainty in the magnitude, profile and slope of the skew makes acquisition of the block difficult.
Further, multitrack implementations present the additional requirement of consistent track alignment in order to correctly process the data through the Error Correction Code (ECC) circuitry. During a read operation, pointers are used to improve the correction power of the ECC decoder. These pointers and the ECC block must be aligned correctly if the correction power of the ECC block is to be maximized.